Measurement of hydrogen peroxide is very crucial in both biomedical and environmental systems. Industries such as plastic and food processing also require hydrogen peroxide. In many biological reaction systems, hydrogen peroxide is a resultant product of several biologically important oxidases. Therefore, it is an important indicator to monitor various biological reactions. Various methods of measuring H.sub.2 O.sub.2 concentration have been developed including fluorometry, fiber-optics, chemiluminescene, and electrochemical methods for aqueous and gaseous samples.
In this invention, an electrode is used as a reactor to generate a desired mixed-valence cluster with proper catalytic property of hydrogen peroxide onto the electrode surface, which serves as a chemical sensor to determine the concentration of H.sub.2 O.sub.2. A mixed-valence cluster is a polynuclear compound with two or more metal clusters which linked by a ligand [D. .A-inverted.. Brown, Mixed-Valence Compounds, D. Reidel Press, Boston 1980]. A typical mixed-valence compound is prepared by mixing aqueous solutions of the anion and cationic metals, which result in the immediate formation (precipitation) of mixed-valence product. In addition to its catalytic property of hydrogen peroxide, it has been found that the electrons are de-localized on the entire complex and the evidence of detecting various redox potential between the two metal centers proves the existence of electronic interactions in the complex. This characteristic of a possible inter-valence charge transfer (IVCT) through the bridging ligand of the complexes can be used as electronic wire [M. D. Ward, Chemical Society Reviews, 1995, 24, 121]. Based on these findings, the electron transfer direction is propagated directionally through bridging ligands at the control of electrode.
Due to a relative high overvoltage requirement and possible interference, a direct amperometric detection scheme for H.sub.2 O.sub.2 seems not feasible for environmental and biological samples. Few years ago an attempt [M. S. Lin, et al., Electroanalysis 1990, 2, 511; M. S. Lin, et al., Anal. Chim. Acta 1990, 234,453.] was made to develop a peroxidase containing system so that H.sub.2 O.sub.2 concentration could be measured, although the sensitivity of the detection is limited. However, the catalyst modified electrode may provide a better solution for this problem. Recently, Wang et al. utilized various carbon materials as transducers that were modified with a series of precious metal such as Pt, Pd, and Rh to reduce the overvoltage for the determination of biological significant through H.sub.2 O.sub.2 in biological systems [J. Wang, and L. Angnes, Anal. Chem. 1992, 64, 456.; Joseph Wang and Qiang Chen, Anal. Chem. 1994, 66, 1007-1011; Joseph Wang, Jie Liu, Liang Chen, and Fang Lu, Anal.Chem. 1994, 66, 3600; Joseph Wang, Fang Lu, L. Angnes, Jie Liu, H. Sakslund, Q. Chen, M. Pedrero, L. Chen, and O. Hammerich, Anal. Chim. Acta, 1995, 305, 3].
The method of monitoring H.sub.2 O.sub.2 concentration by amperometry in the absence of catalyst requires a high overvoltage, which in turn makes it easily interfered by other oxidizable compounds such as ascorbic acid, uric acid, dopamine, cystein, and acetaminophen, in biological systems.
In the past thirty years, various glucose chemical sensors have been invented. Most of them were designed to monitor reactants (such as glucose and O.sub.2) or products (such as H.sub.2 O.sub.2) of an enzymatic reaction. Measurement of glucose concentration by directly catalyzing glucose has an in situ advantage which is no enzyme is needed; however, other hydrocarbon compounds in blood as well as glucose are also catalyzed. Consequently, this method suffers the lack of selectivity. In addition, its monitoring potential (0.5 V) would not prevent the interference from the easily oxidizable compounds in blood.
The most common approach in many glucose biosensor systems was designed to monitor H.sub.2 O.sub.2 concentration, wherein a glucose oxidase was used as an indentifier. By employing an electrode as a transducer in an electrochemical system, an external potential higher than 0.9 V is required to oxidize the H.sub.2 O.sub.2 ; thus, this high potential might result in undesirable interference of oxidation current generated from other biochemically active compounds such as ascorbic acid, uric acid, . . . etc.
In the Joseph Wang, et al. 1990's articles mentioned-above, a series of precious metals (such as rhodium, ruthenium, palladium, . . . etc.) were utilized in the H.sub.2 O.sub.2 monitoring system so that the overvoltage could be reduced. Joseph Wang and Lucio Angnes (1992) electrochemically deposited Rh on a carbon-filament microelectrode surface. Their approach could reduce the H.sub.2 O.sub.2 monitoring potential from +0.9 V to +0.3 V (vs. Ag/AgCl). The response time of the system for the modified electrode was only 3 seconds. The detection limit was 1.times.10.sup.-4 M. In 20 repeated runs the relative standard deviation was 2.1%. However, it was also observed that the electrode signal decreased by 18% after 15 days of operation. Moreover, the interference from ascorbic acid, uric acid, . . . etc. was still not overcome.
A similar experiment was conducted by Joseph Wang and Qiang Chen (1994) by electrochemical depositing a glucose oxidase and Rh on a carbon electrode surface. This approach which employed Rh as a catalyst in the measurement of H.sub.2 O.sub.2 concentration could reduce the monitoring potential from +0.6 V to +0.05 V. This modified electrode could prevent the interference from ascorbic acid, uric acid and the like. This modified carbon electrode could last for 140 days with the same activity.
More recently, Wang et al. in their 1995's article blended rhuthenium into a carbon powder paste and formed a printed carbon electrode, then employed glucose oxidase and phenol on the electrode surface at 0.8 V potential so that polyphenol was formed, and thus glucose oxidase was immobilized on the electrode surface. The H.sub.2 O.sub.2 monitoring potential of this electrode was +0.4 V, i.e. an oxidation current of H.sub.2 O.sub.2 was able to be detected at this potential. However, electrochemically active compounds in blood such as ascorbic acid had a 0.05 V ox potential, uric acid at 0.3 V, and acetaminophen was 0.25 V. Therefore, at +0.4 V glucose monitoring potential may cause a great interference. The authors of this article had used this electrode at a fixed potential of 0.2 V to detect glucose, and found that the detect limit (S/N=3) was about 2.times.10.sup.-4 M, a linear range was upto to 1.5 mM, and the relative standard deviation was 0.9% in 25 repeated runs.